Instruments for measuring the voltage of an AC or DC signal have been in existence for a long time, and they are in common use in a wide variety of fields. Early voltage measuring instruments, commonly known as “volt-ohm meters,” used an analog meter having a pointer with an angular position indicative of the amplitude of a DC voltage. Such meters could also measure the amplitude of AC signals by rectifying the AC signal and scaling the amplitude to correspond to the root-mean-squared (RMS”) value of the signal. Later volt-ohm meters used a digital display, and were substantially more accurate than analog meters.
Although conventional volt-ohm meters could measure the RMS amplitude of an AC signal, the accuracy of such measurement was based on the assumption that the AC signal had a sinusoidal waveshape. Such volt-ohm meters could not accurately measure the RMS amplitude of a non-sinusoidal AC signal. For this reason, true RMS meters were developed using a variety of techniques.
A block diagram of a conventional AC/DC voltage measuring instrument 10 is shown in FIG. 1. The instrument 10 includes a pair of terminals 12, 14 between which the signal to be measured is applied, generally using a test probe. The terminal 14 is coupled to ground, and the terminal 12 is coupled to several devices. More specifically, the terminal 12 is coupled through a resistor 18 to a comparator and logic circuit 20, through a resistor 24 and switch 26 to test node 30, and through a capacitor 32, resistor 34 and a switch 36 to the test node 30. The test node 30 is connected to ground through a resistor 38 to form a voltage divider with one of the resistors 24, 34.
The test node 30 is connected to an analog-to-digital (“A/D”) converter 40 through a switch 42. A digital signal at the output of the A/D converter 40 is applied to a display 44, and/or it may be applied to other circuits or systems (not shown). The test node 30 is also connected to an input of an amplifier 46. An output of the amplifier 46 is coupled through a capacitor 48 to an RMS circuit 50, which provides an accurate measurement of RMS amplitude of a signal applied to its input. An output of the RMS circuit 50 is coupled through a switch 54 to the A/D converter 40.
The voltage measuring instrument 10 has three operating modes, namely a DC measurement mode, an AC measurement mode, and an automatic measurement mode. These modes are selected by a control unit 60 selectively closing the switches 26, 36, 42, 54. The control unit 60 is, in turn, controlled by either a user selection device 62 or an output from the comparator and logic circuit 20, as explained in greater detail below. The control unit 60 is also connected to the display 44 so that it can show the mode that is currently in use. In the DC measurement mode or the AC measurement mode, the control unit 60 causes the display 44 to display whatever mode is selected through the user selection device 62. However, in the automatic measurement mode, the control unit 60 is controlled by the comparator and logic circuit 20 to cause the display 44 to indicate “Automatic” and to also display “AC” if the comparator and logic circuit 20 detects zero crossings and to otherwise display “Automatic” and “DC.”
When the amplitude of a DC voltage is to be measured, the DC measurement mode is selected through the user selection device 62. The selection device 62 then causes the control unit 60 to close the switches 26 and 42 while the switches 36 and 54 remain open. Closing of the switch 26 connects the input terminal 12 to the test node 30 so that the resistor 24 forms a voltage divider with the resistor 38. The amplitude of the voltage at the test node 30 is thus proportional to the amplitude of the signal applied between the input terminals 12, 14. The test node 30 is connected by the closed switch 42 directly to the A/D converter 40. The A/D then outputs a digital signal indicative of the amplitude of the signal applied between the terminals 12, 14, and the display 44 provides the user with an indication of the amplitude of the DC voltage applied between the terminals 12, 14.
When the amplitude of an AC voltage is to be measured, the AC measurement mode is selected, again through the user selection device 62. The selection device 62 then causes the control unit 60 to close the switches 36 and 54 while the switches 26 and 42 remain open. Closing of the switch 36 connects the input terminal 12 to the test node 30 through the capacitor 32 so that only an AC signal is coupled through the resistor 34, which forms a voltage divider with the resistor 38. The amplitude of the voltage at the test node 30 is thus proportional to the amplitude of the AC signal applied between the input terminals 12, 14. The amplitude of this AC signal at the test node 30 is boosted by the amplifier 46 and coupled through the capacitor 48 to the input of the RMS circuit 50. Although the capacitor 32 passes only AC signals, use of the capacitor 48 is desirable to eliminate offsets that are typically generated by the amplifier 46. In the AC measurement mode, the output of the RMS circuit 50 is coupled through the closed switch 54 to the A/D converter 40. The A/D converter 40 thus outputs a digital signal indicative of the RMS amplitude of the AC signal applied between the terminals 12, 14. The RMS amplitude of this signal is then displayed by the display 44.
In the automatic measurement mode, the voltage measuring instrument 10 switches between the DC measurement mode and the AC measurement mode based on the nature of the signal applied between the terminals 12, 14. This function is accomplished by the comparator and logic circuit 20 detecting zero crossings of the signal to be measured. If the circuit 20 detects zero crossings of the signal applied between the terminals 12, 14, it assumes the signal is an AC signal. It therefore sends a corresponding signal to the control unit 60, which closes the switches 36, 54 to place the instrument 10 in the AC measurement mode, as explained above. If the circuit 20 does not detect zero crossings, it assumes the signal applied between the terminals 12, 14 is a DC signal. The circuit 20 therefore sends a corresponding signal to the control unit 60, which closes the switches 26, 42 to place the instrument 10 in the DC measurement mode, as also explained above.
The voltage measuring instrument 10 shown in FIG. 1 works well in many applications, particularly where the signal to be measured is a pure DC signal or a pure AC signal. However, it does not provide accurate results where the signal to be measured is a DC signal with an AC component or an AC signal having a DC offset. If the AC component is sufficiently large in relation to the DC offset, the comparator and logic circuit 20 will detect zero crossings and therefore switch the instrument 10 to the AC measurement mode. However, the capacitors 32, 48 will block the DC component so that only the AC component will be measured. The RMS circuit 50 will therefore produce a digital output that is indicative of the RMS amplitude of only the AC component. Yet the true RMS amplitude is affected by the DC component as well as the AC component.
Another problem develops if the AC component is relatively small in relation to the DC offset. In such case, the comparator and logic circuit 20 will not detect zero crossings and therefore switch the instrument 10 to the DC measurement mode. The instrument 10 will then provide spurious measurements of the DC amplitude of the signal, which will vary depending upon the sample point used by the A/D converter 40. These inconsistent measurements may indicate to the user that the DC voltage is continuously changing when, in fact, it is constant.
There is therefore a need for voltage measuring instrument and method that can provide accurate measurements of the voltage of signals in an Automatic measurement mode, including DC signals with an AC component or AC signals having a DC offset.